1. Field of the Invention
The present invention relates generally to an image forming apparatus, and more particularly to a capacitive sensor that determines amounts of waste toner in a waste toner box.
2. Description of the Related Art
Color Electro Photographic (EP) printers typically include four image forming units that transfer toner either to an intermediate transport module or directly to a sheet that is transported under the image forming units on a transfer member. In either case the EP process generates waste toner that needs to be cleaned off photoconductive members of the image forming units and the transfer member. Some systems have a central waste toner box that collects the waste toner from the photoconductive members and the transfer member. When the waste toner box fills to its maximum capacity, the waste may backup into the image forming units and damage the imaging apparatus. Therefore, a sensor is required to detect levels of waste toner in the waste toner box.
Prior systems have used a torque sensor to measure back pressure on a toner-moving auger, or an optical sensor to detect when the waste toner box is full. However, both the torque and the optical sensor have their problems. While, the torque sensor requires that the waste toner be packed into the waste toner box, the optical sensor requires a wiper to keep the optical path clear of stray toner.
Another sensor used in the prior system is a capacitive sensor that has a capacitor, the capacitance of the capacitor changes as the medium between plates of the capacitor changes from air to waste toner. The system that uses a capacitance sensor requires that the capacitor be calibrated at the factory and the initial calibration value to be saved in a non-volatile memory. The imaging apparatus then determines a delta change in capacitance value to determine the amount of toner within the waste toner box. If non-volatile memory is not available, then the capacitive sensor has to depend on either absolute value or monitor the relative change in capacitance to determine when the waste toner box is full. However, because of the large tolerance in the absolute value of the capacitance, the size of plates of the capacitor, and the distance between the plates, using absolute values can greatly decrease the expected yield of the waste toner box.
Therefore, it would be desirable to have a capacitive sensor that eliminates the need to store the initial value of the capacitance sensor, in a non-volatile memory while maximizing the capacity of the waste toner box.